In conventional paging systems, one or more fixed site radio transmitters broadcast pager addresses as well as messages throughout a radio paging service area. A pager which receives its address and any accompanying message decodes its message and alerts the user to the receipt of the message.
More recently, two-way pagers have been introduced. Such two-way pagers may receive paging messages as described above, and they are also capable of acknowledging receipt of a message. This acknowledgment is typically accompanied by the pager automatically transmitting a short acknowledgment signal back to a fixed site receiver located at a base station.
In order to maintain a reasonable battery life for the two-way pager, the acknowledgment signal it sends is necessarily a low power signal. Despite this limitation on a pager's output power, it is necessary for the base station to reliably receive each acknowledgment signal transmitted by the pagers in its area.
Due to factors such as uneven land contour and the presence of buildings, it is likely that radio transmissions sent by pagers will be reflected or scattered several times before they arrive at the receiving base station. The resulting reflections of the transmitted signal can combine to produce peaks and nulls in the signal which can result in a very weak signal arriving at the receiving base station.
This problem can be somewhat alleviated by increasing the power of the pager's transmitted signal. However, the pager's battery life, and/or battery size, would be adversely affected.
A conventional technique that is sometimes used in the cellular radio telephone industry includes using a three branch diversity system in which a group of three antennas are arranged in a triangular pattern at a base site to receive incoming signals. The desired result is that at least two of the three antennas will always exhibit sufficient diversity gain to overcome the weak signal problem discussed above. The drawback of this approach is primarily economic in that the cost of equipment is higher than can normally be justified for paging systems.
Another technique sometimes used in the cellular radio telephone industry is to use an antenna system with two branch sectored diversity. With this technique, each cell is divided into three equal sectors, with two directional antennas located in each sector, but the cost of implementing this system is also prohibitive for paging systems.